The present invention relates to a combustion method for reducing nitrogen oxides and smoke emission.
The nitrogen oxides (referred to as NOx, hereinafter) produced in the combustion process in various industrial furnaces such as boilers are sorted broadly into "thermal NOx" and "fuel NOx" from the view point of the causes of the generation of NOx. For suppressing the generation of the thermal NOx, various effective methods have been known such as reduction of the flame temperature, reduction of oxygen concentration in the combustion zone and shortening of the stay time of the combustion gas in the combustion zone of high temperature. On the other hand, it is well known that the reduction of oxygen concentration in the combustion zone and the use of fuel having low nitrogen content are effective measures for suppressing the generation of fuel NOx.
From this point of view, there have been proposed and used various combustion techniques for supressing the generation of NOx, e.g. two-staged combustion, flue gas recirculation, increase of the cooling area of the furnace, use of low NOx burner, low-oxygen combustion and off stoichiometric combustion.
However, these conventional techniques are not applicable to all boilers. Namely, it is necessary to select these techniques suitably in accordance with the scale or size of the boiler, construction of the boiler, condition of operation and the like factors. The adoption of these conventional techniques also poses various problems concerning stability of the flame, emission of unburnt substances and smoke, responsive characteristic to the fluctuation of load and thermal efficiency. Further, the application of these conventional techniques to the existing boilers encounters various problems or difficulties such as cost of modification of the boiler, increase of the fuel consumption and so forth.
In the conventional various industrial furnaces, particularly in boilers, it is a current measure to reduce the size of the boiler by adopting a high heat liberation combustion. To this end, it is intended to rapidly mix the fuel injected from the burner with the combustion air to complete the combustion in a shorter period of time.
FIG. 1 is a side elevational sectional view of a construction of a burner and its vicinity in a conventional water-tube type package boiler. Referring to this Figure, the combustion air A supplied from a blower is regulated by a damper 1 and is made to swirl by register vanes 3 in a wind box 2. The swirling flow of the combustion air is then introduced into a combustion chamber 4. On the other hand, the fuel which may be either a gas, liquid or even solid is supplied to a burner 5 and is atomized from a burner tip 7 disposed in the burnertile 6 into the combustion chamber. The fuel is then rapidly mixed with the combustion air, and the combustion is completed in a short period of time while the flame is held by a flame holder 8 such as a swirler provided at the end of the burner 5.
The illustrated combustion system is merely an example, and various other combustion systems have been used. For instance, there have been proposed and used combustion systems having double register vanes, or a combustion system with no register vane to eliminate the swirling of air. However, when it is desired to all of the combustion air, the register vanes are necessarily disposed in the wind box.
In the above-described combustion system, the swirl is given to the combustion air A at a region spaced in the upstream side by a considerable distance from the combustion zone in which the fuel and the air are mixed to make the combustion. Therefore, the swirl has been attenuated as the flow of air reaches the combustion zone. More specifically, the maximum axial component of the velocity of the swirling flow is in inverse proportion to the square of the axial distance from the point at which the swirl is started, and the maximum values of the tangential and radial components are changed also in inverse proportion to the square of the axial distance. Therefore, the swirl is considerably attenuated by the time when it reaches the combustion zone, and is changed into the state of a turbulent flow.
As a result, the combustion is unstabilized particularly when the load on the boiler is low.
It is also pointed out that a so-called edge current is likely to be produced behind the flame holder 8 attached to the end of the burner. Therefore, the disturbance of the air flow is further enhanced. As the fuel is injected to the flow of combustion air in a large scale of turbulence, the fuel is atomized as it penetrates the major flow of air. In consequence, the fuel and air are rapidly mixed uniformly, resulting in a high combustion rate and short flame length. As a result, the generation of NOx is inconveniently increased, although the high heat liberation combustion is achieved.
The rapid mixing of the fuel and air and the increase of generation of NOx attributable to the rapid mixing are notable particularly in case of a flue smoke tube boiler incorporating a diffuser as the flame holder. FIG. 2 is a side elevational sectional view showing the construction around the burner of such a type of boiler. Referring to FIG. 2, the combustion air A supplied from the blower is introduced into the wind box 2, and is divided into a primary air which flows through the diffuser provided at the end of the burner 5 and a secondary air which flows through a swirler 9 and then through the annular space between the diffuser 8 and the inner circle of the swirler 9 along the burnertile 6. The diffuser 8 has radial swirl slits and air ports of about 5 mm dia. formed in its conical wall. These swirl slits and the air ports in combination form a low speed zone of the air to stabilize the flame.
The swirler 9 of the air register has a conical outer surface which is selected from a view point of promotion of rapid combustion. Also, the swirler 9 has a ratio d/D of inside diameter d to the outside diameter D of 0.7 or greater. More specifically, the inside diameter d is the diameter of the inner circle of the swirler 9 in the plane including the conical bottom surface of the diffuser 8, while the outside diameter D is the diameter of the outer circle of the swirler 9 in the same plane, as shown in FIG. 2 I. In addition, the inlet angle .alpha. formed between the vane of the swirler 9 and the ring 4 to which the swirler 9 is attached is 15.degree. or greater. (See FIG. 2 II).
Further, the swirling angle .beta. is selected to fall within the range of between 0.degree. and 60.degree.. This swirling angle .beta. is the angle formed between the vane of the swirler 9 and the axis of the burner as shown in FIG. 2 III. The swirling angle .beta. of 0.degree. provides a straight air flow.
The air flowing through the aforementioned annular section is an axial flow, while the air passed through the swirler 9 constitutes an external swirling flow having an axial, tangential and a radial component. This external swirling flow collides with the above-mentioned axial flow in the burnertile 6 to form a jetting flow having a large scale of turbelency and a large velocity gradient. On the other hand, the fuel is injected from the burner tip 7 into the combustion chamber 4. A smaller part of fuel having small penetration is gently mixed with the primary air to be burnt in a state of rich mixture to stabilize the flame, while the major part of the fuel having a large penetration flows across the primary combustion zone so as to be mixed with the air flow of large scale of disturbance and large velocity gradient, thereby to form a secondary combustion zone. In this secondary combustion zone, the fuel is subjected to a turbulency and diffusion caused by the turbulency of the air flow and the shearing force of the air, and is mixed with the air uniformly to complete the combustion in a short period of time. In consequence, the combustion rate is rendered high, while the flame length is shortened, resulting in a high flame temperature to promote the generation of NOx.